Etch stop layer in integrated circuits

ABSTRACT

An integrated circuit structure includes a dielectric layer and an etch stop layer. The etch stop layer includes a first sub layer including a metal nitride over the first dielectric layer, and a second sub layer overlying or underlying the first sub layer. The second sub layer includes a metal compound comprising an element selected from carbon and oxygen, and is in contact with the first sub layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the following provisionally filedU.S. Patent application: Application Ser. No. 62/065,459, filed Oct. 17,2014, and entitled “Etch Stop Layers and Methods of Making Same;” whichapplication is hereby incorporated herein by reference.

BACKGROUND

In integrated circuit art, a commonly used method for forminginterconnect structures, which include metal lines and vias, is known as“damascene.” Generally, this method involves forming an opening in adielectric layer using photo lithography and etching techniques. Afterthe formation, the opening is filled with copper or copper alloys.Excess copper on the surface of the dielectric layer is then removedthrough a Chemical Mechanical Polish (CMP) process. The remaining copperor copper alloy forms metal vias and/or metal lines.

Damascene processes include dual damascene processes and singledamascene processes. In a dual damascene process, trenches and viaopenings are formed first. The via openings are aligned to conductivefeatures such as metal lines in an underlying layer. The trenches andthe via openings are then filled in a same metal filling process to formmetal lines and vias, respectively. In a single damascene process, metallines or vias, but not both, are formed.

To form via openings in a dielectric layer, an etching process isperformed to expose the underlying metal lines. To prevent excessover-etch that may damage the underlying metal lines, an etch stop layermay be used. The etching process is first stopped on the etch stoplayer, and then a different etching gas/chemical is used to etch-throughthe etch stop layer, so that the underlying metal lines are exposed.Commonly used etch stop materials include silicon nitride, siliconcarbide, silicon carbonitride, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 12 illustrate the cross-sectional views of intermediatestages in the formation of an interconnect structure in accordance withsome embodiments;

FIG. 13 illustrates a cross-sectional view of an interconnect structurein accordance with some embodiments, wherein a metal carbide layer andan overlying metal nitride layer in combination form an etch stop layer;

FIG. 14 illustrates a cross-sectional view of an interconnect structurein accordance with some embodiments, wherein a metal nitride layer andan overlying metal carbide layer or metal oxide layer in combinationform an etch stop layer; and

FIG. 15 illustrates a process flow for forming an interconnect structurein accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Interconnect structures of integrated circuits and the methods offorming the same are provided in accordance with various exemplaryembodiments. The intermediate stages of forming the interconnectstructures are illustrated. The variations of the embodiments arediscussed. Throughout the various views and illustrative embodiments,like reference numbers are used to designate like elements.

FIGS. 1 through 12 illustrate the cross-sectional views of intermediatestages in the formation of interconnect structures of integratedcircuits in accordance with some embodiments. The steps shown in FIGS. 1through 12 are also illustrated schematically in process flow 200 asshown in FIG. 15. In the subsequent discussion, the process steps shownin FIGS. 1 through 12 are also discussed referring to the process stepsin FIG. 15.

FIG. 1 illustrates wafer 100, which includes semiconductor substrate 20and the features formed on a top surface of semiconductor substrate 20.In accordance with some embodiments of the present disclosure,semiconductor substrate 20 is formed of a crystalline semiconductormaterial such as silicon, germanium, silicon germanium, a III-V compoundsemiconductor such as GaAsP, AlinAs, AlGaAs, GaInAs, GaInP, GaInAsP,and/or the like. Semiconductor substrate 20 may be a bulk siliconsubstrate or a Silicon-On-Insulator (SOI) substrate.

In accordance with some embodiments of the present disclosure, wafer 100is used to form a device die. In these embodiments, integrated circuitdevices 22 are formed at the top surface of semiconductor substrate 20.Exemplary integrated circuit devices 22 may include ComplementaryMetal-Oxide Semiconductor (CMOS) transistors, resistors, capacitors,diodes, and the like. The details of integrated circuit devices 22 arenot illustrated herein. In alternative embodiments, wafer 100 is usedfor forming interposers. In these embodiments, no active devices such astransistors and diodes are formed at the surface of substrate 20. Theremay (or may not) be passive devices such as capacitors, resistors,inductors, or the like formed in wafer 100. Substrate 20 may also be adielectric substrate in the embodiments in which wafer 100 is aninterposer wafer. Furthermore, through-vias (not shown) may be formed topenetrate through substrate 20 in order to interconnect the componentson the opposite sides of substrate 20.

Inter-Layer Dielectric (ILD) 24 is formed over semiconductor substrate20 and fills the space between the gate stacks of transistors (notshown) in integrated circuit devices 22. In some exemplary embodiments,ILD 24 comprises phosphosilicate glass (PSG), borosilicate glass (BSG),boron-doped phosphosilicate glass (BPSG), fluorine-doped silicate glass(FSG), tetraethyl orthosilicate (TEOS), or the like. ILD 24 may beformed using spin coating, Flowable Chemical Vapor Deposition (FCVD), orthe like. In accordance with alternative embodiments of the presentdisclosure, ILD 24 is formed using a deposition method such as PlasmaEnhanced Chemical Vapor Deposition (PECVD), Low Pressure Chemical VaporDeposition (LPCVD), or the like.

Contact plugs 28 are formed in ILD 24 and are used to electricallyconnect to integrated circuit devices 22. For example, contact plugs 28may include gate contact plugs that are connected to the gate electrodesof transistors (not shown) in integrated circuit devices 22 andsource/drain contact plugs that are electrically connected to thesource/drain regions of the transistors. In accordance with someembodiments of the present disclosure, contact plugs 28 are formed of amaterial selected from tungsten, aluminum, copper, titanium, tantalum,titanium nitride, tantalum nitride, alloys therefore, and/ormulti-layers thereof. The formation of contact plugs 28 may includeetching ILD 24 to form contact openings, filling a conductivematerial(s) into the contact openings until the conductive materialfills the entireties of the contact openings, and performing aplanarization (such as Chemical Mechanical Polish (CMP)) to level thetop surfaces of contact plugs 28 with the top surface of ILD 24.

Referring to FIG. 2, etch stop layer 26 is formed over ILD 24 andintegrated circuit devices 22, if any. Etch stop layer 26 may comprise ametal nitride, a metal carbide, a metal oxide, and/or the like, whereinthe metal may include aluminum (Al), manganese (Mn), Copper (Cu), ormulti-layers thereof. Etch stop layer 26 may also have a structuresimilar to the structure of the subsequently formed etch stop layer 40(such as what are shown in FIGS. 6, 13, and 14). In accordance withalternative embodiments, etch stop layer 26 includes silicon carbide,silicon nitride, silicon oxynitride, silicon carbo-nitride, or the like.Etch stop layer 26 is formed of a material that has a high etchingselectivity with the overlying dielectric layer 30, and hence etch stoplayer 26 may be used to stop the etching of dielectric layer 30.

Further illustrated in FIG. 2 is dielectric layer 30, which isalternatively referred to as Inter-Metal Dielectric (IMD) layer 30hereinafter. In accordance with some embodiments of the presentdisclosure, IMD layer 30 is formed of a low-k dielectric material havinga dielectric constant (k-value) lower than about 3.0, about 2.5, or evenlower. IMD layer 30 may comprise Black Diamond (a registered trademarkof Applied Materials), a carbon-containing low-k dielectric material,Hydrogen SilsesQuioxane (HSQ), MethylSilsesQuioxane (MSQ), or the like.

Conductive metal lines 32 are formed in IMD 30. The respective step isalso shown as step 202 in process flow 200 shown in FIG. 15. Inaccordance with some embodiments, metal lines 32 include diffusionbarrier layers 34 and copper-containing material 36 over diffusionbarrier layers 34. Diffusion barrier layers 34 may include titanium,titanium nitride, tantalum, tantalum nitride, or the like, and have thefunction of preventing copper in copper-containing material 36 fromdiffusing into IMD 30. Conductive lines 32 are referred to as metallines 32 hereinafter. FIG. 2 illustrates that metal lines 32 are in abottom metal layer, which is the metal layer immediately over contactplugs 28. The illustrated metal lines 32 may also represent metal linesin any metal layer that is over the bottom metal layer.

In accordance with some embodiments of the present disclosure, as shownin FIG. 3, metal capping layers 38 are formed over metal lines 32. Therespective step is also shown as step 204 in process flow 200 shown inFIG. 15. In accordance with some embodiments of the present disclosure,metal capping layers 38 include cobalt (Co), tungsten (W), tantalum(Ta), nickel (Ni), molybdenum (Mo), manganese (Mn), titanium (Ti), iron(Fe), CoWP, CoB, or combinations thereof. Metal capping layers 38 may beformed selectively using electroless plating, during which wafer 100 issubmerged in a plating solution. In alternative embodiments, metalcapping layers 38 are blanket formed on metal lines 32 and IMD layer 30,for example, using Physical Vapor Deposition (PVD), followed by a photolithography process to etch the undesirable portions.

Next, as shown in FIGS. 4 through 6, etch stop layer 40 is formed. Inaccordance with some embodiments of the present disclosure, etch stoplayer 40 includes two or more sub layers formed of metal compounds, witheach of the sub layers alternatively referred to as an etch stop layerhereinafter.

Referring to FIG. 4, etch stop layer 40 a (which is a sub layer of etchstop layer 40 as in FIG. 6) is formed. The respective step is also shownas step 206 in process flow 200 shown in FIG. 15. In accordance withsome embodiments of the present disclosure, etch stop layer 40 aincludes a metal carbide. The metal in etch stop layer 40 a may includeAl, Cu, Mn, or combinations thereof. Accordingly, etch stop layer 40 amay include aluminum carbide, copper carbide, manganese carbide, orcombinations thereof. In some exemplary embodiments, etch stop layer 40a is free from nitrogen, or substantially free (for example, with anatomic percentage smaller than about one percent) from nitrogen. Inalternative embodiments, etch stop layer 40 a further comprisesnitrogen, and hence may include a metal carbo-nitride. The nitrogen inthe metal carbo-nitride may be low, for example, with an atomicpercentage lower than about 10 percent or lower than about 5 percent.Furthermore, etch stop layer 40 a is free from oxygen.

The formation methods of etch stop layer 40 a include, and are notlimited to, CVD and Atomic Layer Deposition (ALD). The thickness Ti ofetch stop layer 40 a is smaller than about 20 Å, and may be in the rangebetween about 5 Å and 20 Å. The bottom surface of etch stop layer 40 ais in contact with the top surfaces of IMD layer 30 and metal cappinglayers 38. Etch stop layer 40 a has a good adhesion to IMD layer 30 andmetal capping layers 38.

Next, as shown in FIG. 5, etch stop layer 40 b (which is also a sublayer of etch stop layer 40 as in FIG. 6) is formed. The respective stepis also shown as step 208 in process flow 200 shown in FIG. 15. Inaccordance with some embodiments of the present disclosure, etch stoplayer 40 b includes a metal nitride. The metal in etch stop layer 40 bmay include Al, Cu, Mn, or combinations thereof. Accordingly, etch stoplayer 40 b may include aluminum nitride, copper nitride, manganesenitride, or combinations thereof. The metal in etch stop layer 40 b maybe the same as the metal in etch stop layer 40 a. Having a same metal inetch stop layers 40 a and 40 b may advantageously improve the adhesionbetween etch stop layers 40 a and 40 b, make the formation processeseasier, and reduce the undesirable interaction between etch stop layers40 a and 40 b. In accordance with alternative embodiments, the metal inetch stop layer 40 b is different from the metal in etch stop layer 40a. The atomic percentages of the metal and nitrogen in etch stop layer40 b may be between about 20 percent and about 80 percent in accordancewith some exemplary embodiments. For example, etch stop layer 40 b mayinclude Al₂N₃ in accordance with some exemplary embodiments.

Etch stop layer 40 b, when deposited, may be free from carbon andoxygen, or substantially free from carbon and oxygen, for example, withan atomic percentage of each of the carbon and oxygen (if any) beinglower than about 1 percent.

The formation methods for forming etch stop layer 40 b include, and arenot limited to, CVD and ALD. The thickness T2 of etch stop layer 40 b issmaller than about 70 Å, and may be in the range between about 5 Å and70 Å. The bottom surface of etch stop layer 40 b may be in contact withetch stop layer 40 a.

Next, as shown in FIG. 6, etch stop layer 40 c is formed. The respectivestep is also shown as step 210 in process flow 200 shown in FIG. 15. Inaccordance with some embodiments of the present disclosure, etch stoplayer 40 c is formed by performing a treatment on etch stop layer 40 b,so that a top surface layer of etch stop layer 40 b is converted intoetch stop layer 40 c. The bottom portion of etch stop layer 40 b, on theother hand, is not converted, and hence remains to be the metal nitridelayer. The treatment may be performed using a carbon-containing processgas such as CH_(x) (with x being an integer such as 1, 2, 4, or 4), CO₂,or the like. During the treatment, wafer 100 may be heated, for example,to a temperature in the range between about 200° C. and about 400° C.The treatment duration may be in the range between about 5 seconds andabout 30 seconds. The treatment may be performed with plasma turned on.Alternatively, the treatment is performed without turning on plasma.

As a result of treating etch stop layer 40 b in the carbon-containingprocess gas, etch stop layer 40 c comprises a metal carbo-nitride.Depending on the metal in etch stop layer 40 b, the metal carbo-nitridemay be aluminum carbo-nitride, copper carbo-nitride, manganesecarbo-nitride, or combinations thereof. In these embodiments, the metalin etch stop layer 40 c is the same type of metal as in etch stop layer40 b. Also, the ratio of the atomic percentage of metal to the atomicpercentage of nitrogen in etch stop layer 40 a is equal to that of 40 b.The thickness T3 of etch stop layer 40 c may be smaller than about 20 Å,and may be between about 5 Å and about 20 Å in accordance with someembodiments. Furthermore, since the top layer of etch stop layer 40 b isconverted into etch stop layer 40 c, the thickness of etch stop layer 40b is reduced from T2 (FIGS. 5) to T4. Thickness T4 may be in the rangebetween about 5 Å and about 50 Å. Furthermore, experiment resultsindicate that when the thickness of metal nitride layer 40 b is about 10Å or slightly lower (as low as about 5 Å), it can still reliably act asan etch stop layer stopping the etching of the overlying low-kdielectric layer. Accordingly, thickness T4 may be between about 5 Å andabout 20 Å (and may be smaller than about 10 Å), so that it can performthe function of stop etching, while its thickness is still small enoughnot to cause significant parasitic capacitance in the resultinginterconnect structure.

In accordance with alternative embodiments of the present disclosure,etch stop layer 40 c is formed through deposition, for example, usingCVD or ALD. Accordingly, etch stop layer 40 c may comprise a metalcarbide, and may be free from nitrogen, or substantially free fromnitrogen (for example, with the nitrogen having an atomic parentagesmaller than about 1 percent). Alternatively, etch stop layer 40 c mayalso be deposited as a metal carbo-nitride. In these embodiments, themetal in etch stop layer 40 c may be the same as, or different from, themetal in etch stop layer 40 b, and thickness T4 may be smaller thanabout 50 Å, smaller than about 10 Å, or between about 5 Å and about 20Å.

In accordance with alternative embodiments of the present disclosure,etch stop layer 40 c is formed by performing a treatment on etch stoplayer 40 b using an oxygen-containing process gas such as O₂. During thetreatment, wafer 100 may be heated, for example, to a temperature in therange between about 200° C. and about 400° C. The treatment duration maybe in the range between about 5 seconds and about 60 seconds. Thetreatment may be performed with plasma turned on. Alternatively, thetreatment is performed without turning on plasma.

As a result of treating etch stop layer 40 b in the oxygen-containingprocess gas, the resulting etch stop layer 40 c comprises a metaloxynitride. Depending on the metal in etch stop layer 40 b, the metaloxynitride may be aluminum oxynitride, copper oxynitride, manganeseoxynitride, or combinations thereof. In these embodiments, the metal inetch stop layer 40 c is the same metal as in etch stop layer 40 b. It isnoted that the metal oxynitride is not used for forming etch stop layer40 a since the oxygen in the metal oxynitride may degrade the ability ofmetal capping layer 38 for preventing electro-migration. During thetreatment using the oxygen-containing process gas, the bottom layer ofetch stop layer 40 b is not converted to metal oxynitride, and remainsto be a metal nitride layer. In these embodiments, the thickness T1, T3,and T4 may be similar to the embodiments in which etch stop layer 40 ccomprises carbon rather than oxygen.

Throughout the description, etch stop layers 40 a, 40 b, and 40 c are incombination referred to as etch stop layer 40. The etch stop layer 40 b(after the formation of etch stop layer 40 c) may be free from carbonand oxygen, or substantially free from carbon and oxygen. For example,an atomic percentage of each of the carbon and oxygen (if any) is lowerthan about 1 percent. In addition, etch stop layer 40 b has acomposition different from the composition of each of etch stop layers40 a and 40 c, wherein either etch stop layer 40 b includes elementsdifferent from the elements of etch stop layers 40 a and 40 c, and/orthe atomic percentages of the elements in etch stop layer 40 b aredifferent from the atomic percentages of the corresponding elements inetch stop layers 40 a and 40 c.

Referring to FIG. 7, IMD layer 42 is formed over etch stop layer 40. Therespective step is also shown as step 212 in process flow 200 shown inFIG. 15. In accordance with some embodiments, IMD layer 42 is formed ofa material that is selected from the same candidate materials forforming IMD layer 30. For example, IMD layer 42 may be formed of acarbon-containing dielectric material, Black Diamond, HSQ, MSQ, or thelike. IMD layer 42 may also have a low-k value, which may be lower thanabout 3.0, 2.5, or 2.0. In accordance with some embodiments of thepresent disclosure, the formation of IMD layer 42 includes depositing aporogen-containing dielectric material and then performing a curingprocess to drive out the porogen, and hence the remaining IMD layer 42is porous.

Referring to FIG. 8, trenches 46 and via opening 44 are formed in IMDlayer 42. The respective step is also shown as step 214 in process flow200 shown in FIG. 15. In accordance with some embodiments of the presentdisclosure, the formation processes includes performing a photolithography process to etch IMD layer 42 in order to form initial viaopenings, wherein the initial via openings extend from the top surfaceof IMD layer 42 to an intermediate level between the top surface and thebottom surface of IMD layer 42. Next, a metal hard mask (not shown) isformed and patterned to define the patterns of trenches 46. Ananisotropic etching is then performed to etch IMD layer 42 to formtrenches 46. At the same time trenches 46 are formed, the via openingsextend down to etch stop layer 40, forming via opening 44 as illustratedin FIG. 8. The etching step for forming trenches 46 may be performedusing a time-mode, and may be stopped after the etching has beenperformed for a pre-set period of time. Other etch and stop pointdetection techniques, however, are also contemplated. In alternativeembodiments, via opening 44 and trenches 46 are formed in separate photolithography processes. For example, in a first photo lithographyprocess, via opening 44 are formed to extend all the way down to etchstop layer 40. In a second lithography process, trenches 46 are formedto extend to an intermediate level of IMD 42. Etch stop layer 40 is thenetched to expose the underlying metal capping layers 38.

In accordance with some embodiments of the present disclosure, theetching of IMD layer 42 is performed using a process gas comprisingfluorine and carbon, wherein fluorine is used for etching, while carbonforms a polymer protecting the sidewalls of the resulting via opening 44and trenches 46. With an appropriate fluorine-to-carbon ratio, viaopening 44 and trenches 46 may have desirable profiles. For example, theprocess gases for the etching include a fluorine and carbon containinggas(es) such as C₄F₈ and/or CF₄ and a carrier gas such as N₂. Inalternative embodiments, the process gases for the etching include CH₂F₂and a carrier gas such as N₂.

The etching is performed using etch stop layer 40 to stop the etching.In accordance with some embodiments, via opening 44 penetrates throughetch stop layer 40 c and stops on etch stop layer 40 b. The metalnitride in etch stop layer 40 is good at stopping the etching of IMDlayer 42. Accordingly, although etch stop layer 40 b is very thin,sometimes several angstroms to tens of angstroms thick, it may stilleffectively stop the etching.

FIG. 9 illustrates the etching of etch stop layers 40 b and 40 a. Afterthe etching of IMD layer 42, the process gas is changed to the processgas for etching through etch stop layer 40 b and etch stop layer 40 a,so that metal capping layer 38 is exposed to the resulting via opening44.

FIG. 10 illustrates the formation of conductive via 48 in via opening 44(FIG. 9), and conductive lines 50 in trenches 46. The respective step isalso shown as step 216 in process flow 200 shown in FIG. 15. Via 48 andconductive lines 50 may include liners 52, such as diffusion barrierlayers, adhesion layers, or the like. Liners 52 may include titanium,titanium nitride, tantalum, tantalum nitride, or other alternatives. Theinner material of conductive lines 50 over liners 52 is a conductivematerial such as copper, a copper alloy, nickel, gold, tungsten,aluminum, or the like. In some embodiments, the formation of via 48 andconductive lines 50 includes performing a blanket deposition to formliner 52, depositing a thin seed layer of copper or copper alloy, andfilling the rest of via opening 44 and trenches 46 through, for example,electro-plating, electro-less plating, deposition, or the like. A CMP isperformed to level the surface of conductive lines 50 and/or liners 52,and to remove excess material from the surface of IMD layer 42.

FIG. 11 illustrates the formation of metal capping layers 54 overconductive lines 50. Metal capping layers 54 may be formed of a materialselected from the same group of candidate materials for forming metalcapping layer 38. Furthermore, Metal capping layers 54 may be formedusing the same method for forming Metal capping layers 38.

In a subsequent step, as shown in FIG. 12, etch stop layer 56 is formedover metal capping layers 54 and IMD layer 42. The structure, thematerials, and the formation methods of etch stop layer 56 are similarto that of etch stop layer 40, and hence are not repeated herein.Process may then be continued to form more features such as IMD layers,metal lines, vias, and the like, or the structure in FIG. 12.

FIGS. 13 and 14 illustrate wafers 100 including the interconnectstructures in accordance with alternative embodiments. Unless specifiedotherwise, the materials and the formation methods of the components inthese embodiments are essentially the same as the like components, whichare denoted by like reference numerals in the embodiments shown in FIGS.1 through 12. The details regarding the formation process and thematerials of the components shown in FIGS. 13 and 14 may thus be foundin the discussion of the embodiments shown in FIGS. 1 through 12.

The structure shown in FIG. 13 is similar to the structure shown in FIG.11, except that etch stop layer 40 c (FIG. 12) is not formed in theseembodiments. Etch stop layers 40 a and 40 b are formed. Etch stop layer40 a is in contact with the underlying metal capping layers 38 and IMDlayer 30. Etch stop layer 40 b is in contact with the overlying IMDlayer 42 and via 48.

The structure shown in FIG. 14 is also similar to the structure shown inFIG. 11, except that etch stop layer 40 a (FIG. 12) is not formed inthese embodiments. Etch stop layers 40 b and 40 c are formed. Etch stoplayer 40 b is in contact with the underlying metal capping layers 38 andIMD layer 30. Etch stop layer 40 c is in contact with the overlying IMDlayer 42 and via 48.

The embodiments of the present disclosure have some advantageousfeatures. Some metal nitrides can be good etch stop layers that caneffectively stop the etching of overlying IMD layers. Accordingly, whenused as etch stop layers, these metal nitrides can be formed very thinwithout sacrificing its ability for stop etching. With the etch stoplayers formed of metal nitrides being very thin, the parasiticcapacitance caused by the etch stop layers can be significantly reduced.This is beneficial for small scale integrated circuits forming using 16nm technology or under, in which the parasitic capacitance caused byetch stop layers cannot be ignored. Metal nitrides, however, may havepoor adhesion to low-k dielectric materials, and hence may incurproblems such as delamination when used as etch stop layers. By formingthe metal carbo-nitride, metal nitride, or metal oxynitride overlyingand/or underlying the metal nitride layer, the adhesion problem issolved since the metal carbo-nitride, metal nitride, or metal oxynitridehave good adhesion to both metal nitride and low-k dielectric layer. Theoverall thickness of the multi-layer etch stop layer is still small, andhence the resulting parasitic capacitance is small. As a comparison, theconventional etch stop layers formed of silicon nitride, siliconcarbide, silicon carbonitride, and the like typically need thicknessesgreater than 100 Å in order to effectively stop etching. Thecorresponding parasitic capacitance is thus high.

In accordance with some embodiments of the present disclosure, anintegrated circuit structure includes a dielectric layer and an etchstop layer. The etch stop layer includes a first sub layer including ametal nitride over the first dielectric layer, and a second sub layeroverlying or underlying the first sub layer. The second sub layerincludes a metal compound comprising an element selected from carbon andoxygen, and is in contact with the first sub layer.

In accordance with alternative embodiments of the present disclosure, anintegrated circuit structure includes a first low-k dielectric layer andan etch stop layer. The etch stop layer includes a first sub layercomprising a metal carbide, a second sub layer over the first sub layer,wherein the second sub layer comprises a metal nitride, and a third sublayer overlying the second sub layer, wherein the third sub layercomprises a metal compound comprising an element selected from carbonand oxygen. A second low-k dielectric layer is over the etch stop layer.A via includes a portion in the second low-k dielectric layer, whereinthe via penetrates through the etch stop layer.

In accordance with yet alternative embodiments of the presentdisclosure, a method includes forming an etch stop layer over a firstdielectric layer, which includes forming a metal nitride layer over thefirst dielectric layer, and performing a treatment on the metal nitridelayer using an oxygen-containing gas or a carbon-containing gas. A topsurface layer of the metal nitride layer is converted to a second sublayer of the etch stop layer, and a bottom layer of the metal nitridelayer remains untreated to act as a first sub layer of the etch stoplayer. The method further includes forming a second dielectric layerover the etch stop layer, and etching the second dielectric layer,wherein the etching stops on the etch stop layer, and etching throughthe etch stop layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An integrated circuit structure comprising: afirst dielectric layer; and an etch stop layer comprising: a first sublayer comprising a metal nitride over the first dielectric layer; and asecond sub layer overlying or underlying the first sub layer, whereinthe second sub layer comprises a metal compound comprising an elementselected from carbon and oxygen, and is in contact with the first sublayer.
 2. The integrated circuit structure of claim 1, wherein thesecond sub layer is underlying the first sub layer, and the second sublayer comprises carbon.
 3. The integrated circuit structure of claim 2,wherein the second sub layer is substantially free from oxygen.
 4. Theintegrated circuit structure of claim 1, wherein the second sub layer isoverlying the first sub layer.
 5. The integrated circuit structure ofclaim 4, wherein the second sub layer comprises carbon.
 6. Theintegrated circuit structure of claim 4, wherein the second sub layercomprises oxygen.
 7. The integrated circuit structure of claim 4 furthercomprising a third sub layer underlying the first sub layer, wherein thethird sub layer comprises a metal carbide.
 8. The integrated circuitstructure of claim 1, wherein the second sub layer further comprisesnitrogen.
 9. The integrated circuit structure of claim 1 furthercomprising: a first conductive line in the first dielectric layer; ametal capping layer over the first conductive line, with the etch stoplayer over and contacting the metal capping layer and the firstdielectric layer; a second dielectric layer over the etch stop layer;and a via comprising a portion in the second dielectric layer, whereinvia further penetrates through the etch stop layer to electricallycouple to the metal capping layer.
 10. An integrated circuit structurecomprising: a first low-k dielectric layer; an etch stop layercomprising: a first sub layer comprising a metal carbide; a second sublayer over the first sub layer, wherein the second sub layer comprises ametal nitride; and a third sub layer overlying the second sub layer,wherein the third sub layer comprises a metal compound comprising anelement selected from carbon and oxygen; a second low-k dielectric layerover the etch stop layer; and a via comprising a portion in the secondlow-k dielectric layer, wherein the via penetrates through the etch stoplayer.
 11. The integrated circuit structure of claim 10, wherein thethird sub layer comprises a metal carbide.
 12. The integrated circuitstructure of claim 11, wherein the third sub layer comprises a metalcarbo-nitride.
 13. The integrated circuit structure of claim 10, whereinthe second sub layer has a composition different from compositions ofthe first sub layer and the third sub layer.
 14. The integrated circuitstructure of claim 10, wherein the second sub layer has a thicknesssmaller than about 50 Å.
 15. A method comprising: forming an etch stoplayer over a first dielectric layer, wherein the forming the etch stoplayer comprises: forming a metal nitride layer over the first dielectriclayer; and performing a treatment on the metal nitride layer using anoxygen-containing gas or a carbon-containing gas, wherein a top surfacelayer of the metal nitride layer is converted to a second sub layer ofthe etch stop layer, and a bottom layer of the metal nitride layerremains untreated to act as a first sub layer of the etch stop layer;forming a second dielectric layer over the etch stop layer; etching thesecond dielectric layer, wherein the etching stops on the etch stoplayer; and etching through the etch stop layer.
 16. The method of claim15, wherein the forming the etch stop layer further comprises: beforethe forming the metal nitride layer, forming a third sub layer of theetch stop layer over and contacting the first dielectric layer, with themetal nitride layer being over and contacting the third sub layer,wherein the third sub layer comprises a metal carbide.
 17. The method ofclaim 16, wherein the third sub layer and the metal nitride layercomprise a same metal.
 18. The method of claim 16, wherein the treatmentis performed using the carbon-containing gas, and the second sub layercomprises a metal carbo-nitride.
 19. The method of claim 16, wherein thetreatment is performed using the oxygen-containing gas, and the secondsub layer comprises a metal oxynitride.
 20. The method of claim 16,wherein the metal nitride layer is formed as substantially free fromcarbon and nitrogen.